The invention relates generally to cardiac pacers, and more particularly to cardiac pacing systems having means for avoiding competition with spontaneous depolarization.
The physical characteristics of the human heart lend themselves to various interactive artificial pacing systems. There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The atria are smaller antechambers which contract in a separate action that precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay, approximately one-eighth of the cardiac cycle. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation then enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles.
Electrical signals corresponding to the contractions appear in the electrocardiagram. A small signal known as the P-wave accompanies atrial contraction while a much larger signal, known as the QRS complex, with a normally predominant R-wave, accompanies the ventricular contraction. Repolarization prior to the next contraction is marked by another small signal in the electrocardiagram known as the T-wave. The P and R-waves can be very reliably detected as timing signals by electrical leads in contact with the respective heart chambers. The typical implanted cardiac pacer operates by supplying missing stimulation pulses on a pacing lead attached to the right ventricle. The R-wave can be sensed by the same lead. An additional lead contacts the left atrium to sense P-waves, if desired.
One of the problems treated by cardiac pacers is heart block caused by impairment of the ability of the bundle of His to conduct normal excitation from the atrium to the ventricle. It has long been apparent that in treating this form of heart disease, it is desirable to base the stimulation of the ventricles on the umimpaired P-wave cycle. This synchronization maintains the heart's normal physiological pacing pattern. Thus, the sino-atrial node, which governs the interval between atrial depolarizations (i.e., the atrial rate) according to the body's needs, controls the artificial ventricular rate in the normal manner.
It is also well known that ventricular stimulation should not be applied during the repolarization period following ventricular contraction. This period begins with the QRS complex and lasts through the T-wave (Q-T period), approximately 300 ms or three-eighths of the cardiac cycle. Stimulation during the Q-T transition can induce undesirable heart rhythms. A spontaneous ventricular beat can arise through normal AV conduction or spuriously as in ectopic ventricular activity. In the latter case, the ventricular beat does not have the normal relationship to atrial excitation.
In an asynchronous (fixed rate) pacer, it is possible that stimulation pulses will be applied to the Q-T period following a spontaneous ventricular beat, whether normally or abnormally conducted. When this happens, stimulation pulses are said to be competing with the natural activity of the heart.
In P-wave synchronized pacers, such as the "Omni-Atricor" manufactured by Cordis Corporation, the assignee of the present application, the normally conducted beat occurs simultaneously with ventricular stimulation in a noncompetitive fashion. Nevertheless, P-wave synchronized ventricular stimulation can compete with ectopic ventricular activity.
Systems for inhibiting the output of an AV synchronous pacer due to spontaneous ventricular signals have been proposed. U.S. Pat. No. 3,903,897 includes means for sensing activity on the ventricular lead and producing a Q-T logic output to a gate which controls the application of stimulation pulses to the ventricular lead. One of the problems of ventricular sensing is that noise appearing on the ventricular lead may falsely inhibit stimulation. The consequences of omitting needed stimulation can outweigh the consequences of competitive pacing. U.S. Pat. No. 3,648,707 describes a notch filter in the ventricular sensing circuitry which apparently excludes 60 Hertz signals. Unfortunately, noise is not confined to 60 Hertz but is present in our environment in varying degrees throughout the electromagnetic spectrum. In addition to the ubiquitous 60 Hertz noise, auto ignitions and hair dryers, for example, produce strong local electromagnetic interference with pulse repetition frequencies as low as 50 Hertz.
Patients without normal atrial activity, as in symptomatic bradycardia, have a need for atrial stimulation as well as ventricular stimulation which alone achieves about 75% of the combined volume flow. So-called bifocal pacers have been proposed for stimulating the atria and the ventricles, for example, in U.S. Pat. Nos. 3,747,604 and 3,783,878. While these systems are apparently capable of stimulating both chambers of the heart, they are based on ventricular timing; that is, unlike physiological pacing, the sensing of ventricular activity resets the timing mechanism.
For a pacer manufacturer, one of the rarely achieved goals in pacer development is to employ without modification circuitry which has been tested and used before for other pacers manufactured by the same company. To ensure reliability, components are usually tested and evaluated over a lengthy product qualification period, and to the extent that a new design incorporates existing circuitry its reliability for life-supporting cardiac pacers can be more accurately estimated in advance.